The present invention relates to the biomedical arts for introducing electrical signals on nerve trunks. The present invention finds particular application in inducing a stream of artifically generated antidromic pulses on the nerve trunk for collision blocking orthodromic pulses moving in the opposite direction on the nerve trunk and will be described with particular reference thereto. It is to be appreciated, however, that the invention may have broader applications including generating action potentials on nerve trunks for other purposes and monitoring naturally occuring nerve impulses.
Heretofore, various techniques have been used to block nerve pulses passing along a nerve trunk. Commonly, an electrode cuff including a dielectric sleeve and three symmetric electrodes were positioned around a nerve trunk. The three electrodes were arranged symmetrically within the sleeve, with an annular cathode positioned in the center and a pair of annular anodes were positioned to either side. A signal generator was connected with the electrodes to apply an electrical pulse train that induced action potentials on the nerve trunk.
One blocking technique was the application of DC currents on the nerve trunk. However, it has been found that the application of DC and unidirectionally pulsed currents induced nerve damage.
To eliminate the DC current induced nerve damage, others have used a bipolar current wave forms such that the average electrical charge passed through an electrode is approximately zero. In one technique, a train of pulses was applied to the cuff electrodes. Each pulse of an exemplary pulse train included a rapid rise to a preselected amplitude, a 200 to 1000 microsecond plateau, and an exponential decay back to zero. The ends of the cuff have been defined as the "arrest" end and the "escape" end. No action potential was intended to emerge from the "arrest" end and the colliding pulse emerges from the "escape" end. This pulse train induced artifically generated antidromic pulses traveling unidirectionally on the nerve trunk. The antidromic pulses collided with and blocked orthodromic pulses traveling in the other direction. To eliminate nerve damage that may result from monopolar stimulation, a relatively long duration, low amplitude rectangular pulse of opposite polarity was applied between each pulse of the first polarity pulse train.
Although it was intended that current should flow within the cuff from the anodes to the cathode, some current (secondary current) also flowed outside the cuff in the body tissue, creating a virtual cathode along the nerve outside the cuff. This secondary current tended to generate unwanted action potentials near the arrest end of the cuff that traveled along the nerve trunk in the orthodromic or undesired direction.
The present invention contemplates a new and improved apparatus and method for generating action potentials which eliminates virtual cathodes.